1. Field of the Invention
This invention relates to methods and apparatus for generating waveforms.
2. Description of the Related Art
There are many known techniques currently employed for generating waveforms, One technique is that of frequency synthesis which may involve the direct or indirect generation of a sine wave at a required frequency from a reference derived from a crystal oscillator, Direct frequency synthesis employs multiplication, mixer and divider circuits, whereas indirect synthesis employs phase-locked loop techniques. Both methods are therefore somewhat complicated to realize. Alternatively, digital forms of frequency synthesis obviate the need for complicated analogue circuitry but require large digital memories, A reference clock frequency is also required for this method in which a number representing the phase change per clock period is shifted into an accumulator on each clock cycle. The accumulator's output is an approximation of a linear phase versus time function which is then converted to analogue form by a digital to analogue converter (DAC), The reader is referred to "Frequency Synthesis by Phase Lock" by W. F, Egan (J. Wiley 1981) for detailed descriptions of synthesiser circuitry.
Other less complicated yet less precise and stable waveform generator circuits are described in many standard electronic text books. For example, a square and triangular wave oscillator can be simply realized using a two-operational amplifier circuit.
In a further example, an integrated circuit including a counter generates square waves which can be converted to an approximate sine wave by passing the square-wave signal through a fifth order band-pass filter.
Some dedicated circuits employ digital Fourier techniques, i.e. a digital expander expands a square wave into a variety of digital forms. These are summed to give a staircase approximation to the desired sine wave output, which is smoothed by a low-pass filter.